Regarding exothermic electronic parts such as power devices, double-side heat dissipation transistors, thyristors, CPU and the like, efficient heat dissipation during their use is important. Generally, conventional measures for such heat dissipation were to (1) improve thermal conductivity of the insulating layer of a printed-wiring board onto which the exothermic electronic parts are to be mounted, and (2) mount the exothermic electronic parts or the printed-wiring board having the exothermic electronic parts mounted thereon onto a heat sink via a thermal interface materials having electric insulation. As the insulating layer of the printed-wiring board and the thermal interface materials, heat dissipating member obtained by curing silicone resin and epoxy resin added with ceramics powder is used.
In recent years, higher speed and higher integration of the circuit in the exothermic electronic parts and higher density of the exothermic electronic parts being mounted onto the printed-wiring board have lead to higher heat generation density and more precise structure in the electronic devices. Accordingly, heat dissipating member having even higher thermal conductivity, being thin and having high strength, has been required.
In addition, with respect to the usage in power modules such as elevators, vehicles, hybrid cars and the like, ceramics substrates made of alumina, beryllia, silicon nitride, aluminum nitride and the like are used in view of thermal conductivity, cost, safety and the like. These ceramics substrates are bonded to a metal circuit plate and to a heat dissipation board and are used as the circuit board. Due to superior insulation characteristics and heat dissipation characteristics with respect to metal substrates having a resin substrate or a resin layer as the insulating material, they are used as the substrate for installing high heat dissipation electronic parts. In recent years, higher integration, higher frequency, higher output and the like of the semiconductor devices have lead to increase in the calorific value from the semiconductor devices. Accordingly, ceramics substrates of aluminum nitride sintered body and silicon nitride sintered body having high thermal conductivity have been used. In particular, aluminum nitride substrate has high thermal conductivity compared with the silicon nitride substrate, and thus it is suitable as the ceramics circuit board for installing high heat dissipation electronic parts.
However, although the aluminum nitride substrate possesses high thermal conductivity, its mechanical strength and toughness are low. Accordingly, there are disadvantages in that breakage occurs by the tightening in the assembly process, cracks easily occur when subjected to thermal cycle, and the like. In particular, when applied for the power modules used under severe load and thermal conditions, such as vehicle, electric railroad, machining tool, robots and the like, these disadvantages become remarkable. Therefore, as the ceramics substrate for installing electronic parts, improvement in mechanical reliability is required, and thus silicon nitride substrate having superior mechanical strength and toughness, although inferior to aluminum nitride substrate in thermal conductivity, has been attracting attention. On the other hand, such ceramics circuit board is higher in cost than the resin substrate or the metal substrate having resin layer as the insulating material, and thus the utilization of the ceramics circuit board is limited.
From the afore-mentioned circumstances, hexagonal boron nitride having superior properties as the electric insulating materials such as (1) high thermal conductivity, (2) high electric insulating property and the like have been receiving attention. However, while the thermal conductivity of boron nitride is 100 W/(m·K) in the in-plane direction (direction in the a-axis), the thermal conductivity in the thickness direction (direction in the c-axis) is 2 W/(m·K). Accordingly, anisotropy of the thermal conductivity derived from the crystalline structure and the flake shape is large. Accordingly, for example, when the thermal interface materials is manufactured, the in-plane direction (direction in the a-axis) of the boron nitride particle and the thickness direction of the thermal interface materials can come vertical with each other, thereby resulting in cases where the high thermal conductivity of the boron nitride in the in-plane direction (direction in the a-axis) could not be utilized sufficiently. On the other hand, high thermal conductivity of the boron nitride particles in the in-plane direction (direction in the a-axis) can be achieved by allowing the in-plane direction (direction in the a axis) of the boron nitride particles and the thickness direction of the thermal interface materials to come in parallel with each other, however, there is a problem in that the material is weak in the tensile stress in the thickness direction.
In Patent Literature 1, a resin composite material containing high rigidity particles such as ceramics and metals by 40 to 90% by volume fraction, the high rigidity particles being in contact with each other three-dimensionally, and a manufacturing method thereof are disclosed. Here, it is also disclosed that such resin composite material can be suitably used in mechanical parts such as a slide member exemplified as a wire saw roller and a gear wheel.
In addition, Patent Literature 2 discloses a sintered ceramics member having at least forsterite and boron nitride as its main component and having boron nitride aligned in one direction, a probe holder formed by using the ceramics member, and a manufacturing method of the ceramics member. Here, it is also disclosed that such ceramics member can be suitably used as a material for a probe holder, into which a probe is inserted. Here, the probe is for a micro contactor used in inspection of semiconductors and liquid crystals, and electrically connects a circuit structure to be inspected and a circuit structure which output signals for inspection.
Patent Literature 3 discloses a method in which a filler having a large anisotropy in shape or in thermal conductivity is mixed and dispersed in a thermosetting resin material, followed by curing of the thermosetting resin. Subsequently, the cured thermosetting resin is crushed, followed by mixing the thermosetting resin dispersed with the filler and a thermoplastic resin to obtain a resin composition for a mold, and then the resin composition is heated to soften and mold the resin composition into a desired shape.
Patent Literatures 4 and 5 disclose of a method for manufacturing a substrate for a printed circuit board comprising the step of impregnating a thermal setting resin (II) into an inorganic continuous porous body selected from the group consisting of an aluminum nitride-boron nitride composite body (AIN—BN), an alumina-boron nitride composite body (Al2O3—BN), a zirconium oxide-boron nitride composite body (ZrO2—BN), silicon nitride-boron nitride composite body (Si3N4—BN), hexagonal boron nitride (h-BN), β-wollastonite (β-CaSiO3), mica, and volcanic soil; and curing to obtain a cured plate body. In addition, it is also disclosed that such substrate for a printed circuit can be suitably used as a substrate for high frequency usage and for directly mounting a semiconductor chip.
Patent Literature 6 discloses a porous body of B—C—N system having a graphite three-dimensional skeletal structure synthesized from a porous polyimide sheet as a starting material, and a heat dissipating material obtained by impregnating a resin in the porous portion to obtain a composite material. Such heat dissipating material has a smaller heat resistance compared with those obtained by impregnating a resin in an ordinary carbon porous body, and an insulating composite material can be obtained by the conversion of the porous body into h-BN. Accordingly, the heat dissipating material is a promising material as a cooling material for electronic parts which require low heat resistance and electric insulating property.